Excitatory amino acids, such as glutamate and aspartate, are prominent neurotransmitters in the mammalian central nervous system. This project employs the hippocampal formation of the rat brain as a test system with which to investigate the mechanisms of excitatory amino acid-mediated transmission. Electrophysiological studies indicate that L-proline acts as an excitant on hippocampal pyramidal cells. In addition, proline-induced excitations exhibit an excitatory amino acid-like pharmacology and proline is released from rat hippocampal slices in a Ca2+-dependent manner. Thus proline, although lacking the second acidic group possessed by recognized amino acid excitants, may function not only as an endogenous excitant but possibly also as a transmitter or modulator in the hippocampal formation. To identify possible "prolinergic" pathways, biochemical markers for proline (Ca2+-dependent release, Na+-dependent high affinity uptake, synaptosomal content, synthetic enzymes) will be related to specific fiber tracts with use of lesion techniques. Parallel studies will compare the pharmacology of proline-induced excitation in the hippocampal slice to the pharmacology of synaptic transmission. Proline receptors on hippocampal synaptic membranes will be labeled with L-(3H)proline, characterized and localized autoradiographically. The proline binding assay will be employed to study receptor mechanisms and to screen analogues for receptor activity. N-Methyl-D-aspartate-sensitive and quisqualate-sensitive excitatory amino acid receptors will be labeled with L-(3H)-glutamate. The effect of NMDA receptor agonists on adenylate cyclase and polyphosphoinositide phosphodiesterase activities will be tested to determine whether the inhibition of glutamate binding to NMDA-sensitive sites by guanine nucleotides implies a guanine nucleotide-dependent link between receptor and enzyme. Possible alterations in excitatory amino acid receptor density or localization in the kindling model of temporal lobe epilepsy will be assessed autoradiographically. Findings from these studies will be relevant to the treatment of neurological conditions, such as epilepsy, which involve hyperactivity of the hippocampal formation, as well as to the etiology of seizures and mental retardation frequently associated with hyperprolinemia.